They are most likely on iOS 6.x.x. That being said, at some point they will come pre-loaded with iOS 7. If you're going to purchase an iPad to get iOS 6 specifically, try and do it locally where you can verify what iOS is on the device in the store (Best Buy should let you do this).
Beware that to reset your device under iOS 6 and *keep it at iOS 6*, you cannot do a restore from iTunes on the PC since it will automatically update it to what Apple is signing as current for the device (which is of course, iOS 7.x.x). To restore as new, you need to restore from the iPad itself (actually an erase) under Settings>General>Reset>Reset All Content and Settings. This will retain the iOS version that is on the device.
Another thing is that you won't be able to update to the most recent iOS 6 (iOS 6.1.3) for the iPad. If it came with iOS 6.0, you are stuck at iOS 6.0...anything above that (but of course not including iOS 7) is no longer signed by Apple nor is considered valid for certain devices.
Overall, it may take some work on your part to keep your device at iOS 6.x.x...I have been doing it since iOS 7 was released.
Also be aware that you may get an auto-pushed download for iOS 7
Work-around to keep the update file off of the device (it will otherwise rob you of 3.1 GB of your storage space unless you install it):
If you get the update file auto-pushed to your device and want to remove it:
*How to keep the device from auto-downloading the update again, even after it's successfully removed:
In your router's settings, set up a block (using Access Restrictions or similar in the router's web interface) to mesu.apple.com. This will prevent the devices from "phoning home" to Apple, checking for the update and getting the download pushed to it again (it WILL get pushed again to the device, even if you remove it).
In the future to check for updates on the device, simply disable the block created above. Remember to reenable the Access Restriction afterwards.